This application is a Divisional application of application Ser. No. 11/752,966, filed May 24, 2007, now ABN, the contents of which are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to an etching composition used for etching conductive films which comprises zinc oxide as the main component and are used for display devices such as liquid crystal displays (LCD) and electroluminescence displays and to an etching process using the etching composition.
2. Description of the Related Art
As the conductive film for display devices such as liquid crystal displays and electroluminescence displays, films of indium tin oxide (ITO) and films of indium zinc oxide (IZO) are widely used. Due to the decrease in the resources for indium, electrically conductive materials which can be used in place of ITO and IZO have been developed.
Conductive films containing zinc oxide as the main component are widely used as the Piezoelectric thin films, optical waveguide films and gas sensitive films and are expected as the material which can be used in place of conventional conductive films for display devices. The conductive film containing zinc oxide as the main component can be supplied at a lower price with greater stability than indium-based conductive films since zinc is used as the raw material. Aluminum and gallium are often added to the zinc oxide films for display devices to decrease electric resistance.
The above conductive films are formed on a substrate such as a glass substrate in accordance with a process for film formation such as the sputtering process and the ion plating process. Then, etching is conducted using a resist or the like as the mask, and an electrode pattern is formed. The etching process includes wet processes and dry processes. An etching composition is used in the wet processes.
It is widely known that zinc oxide is soluble in acids and alkalis. When a transparent electrode of a display device such as a liquid crystal display is formed, the following properties are required for the etching composition:
(1) A great accuracy of working
(2) A suppressed amount of residues of etching
(3) A suitable etching rate
(4) A suppressed change in the etching property with dissolution of zinc oxide
(5) No corrosion of materials such as wiring materials
The related arts will be described more specifically with reference to Figures. FIG. 1 shows a sectional view of a laminate which is obtained by forming a zinc oxide film 2 on a glass substrate 1, followed by forming a resist 3 patterned in accordance with the photolithography. FIG. 2 shows a sectional view of a laminate obtained after etching the laminate shown in FIG. 1 using the resist 3 as the mask. The great accuracy means that the zinc oxide film is etched accurately to give the dimensions shown by the resist pattern. It is preferable that the distance from the end portion of the resist (a) to the end portion of zinc oxide (b) (the amount of the side etching) is decreased as much as possible.
Residues 4 left remaining after the etching cause formation of the leak current between the electrodes when the residues have the electric conductivity. The residues 4 may cause a decrease in adhesion with a film formed above the zinc oxide film 2. Therefore, the presence of the residues is not preferable.
An excessively great etching rate is not preferable since the control of the etching becomes difficult. An etching rate of about 500 to 2,000 angstrom/min is preferable.
The concentration of zinc in the etching composition increases with the progress of the etching. Therefore, it is very important for etching of transparent electrodes in the industrial production that the etching composition is a stable fluid having the etching properties such as the etching rate hardly affected by the concentration of zinc in the etching composition. When the stable etching composition having the etching properties hardly affected by the concentration of zinc is used, the amount of etching of zinc oxide can be further increased.
In Japanese Patent Application Laid-Open No. 8(1996)-330692, a process for forming an electrode pattern for display devices containing zinc oxide as the main component is described. It is described that etching can be conducted with excellent accuracy without formation of residues by the etching using hydrochloric acid or nitric acid as an acidic solution of 0.005 to 0.2 N. However, when the above etching composition is actually used, the accuracy is insufficient due to a great amount of the side etching, and residues are formed. Moreover, the etching rate changes to a great degree due to the change in the concentration of the acid caused by vaporization of water and dissolution of zinc during the etching, and the stable etching cannot be expected.
In Japanese Patent No. 3258780, an electroluminescence element for displays and a process for producing the element are described. It is described that a transparent conductive film can be formed by etching a zinc oxide film with acetic acid. Although the etching of a zinc oxide film with an aqueous solution of acetic acid can be conducted, this process is not preferable due to the formation of residues.
In Japanese Patent No. 3345408, an etching composition for transparent electrodes comprising oxalic acid and a polysulfonic acid compound is disclosed. This composition relates to ITO films, and no descriptions on the etching of a zinc oxide film can be found at all. When a zinc oxide film is etched with an aqueous solution of oxalic acid, hardly soluble crystals of zinc oxalate are formed on the substrate and, therefore, this process is not preferable. Even when a polysulfonic acid compound is added to the aqueous solution of oxalic acid, crystals of zinc oxalate cannot be removed by etching.
In Solid-State Sensor Actuator Workshop (New York: IEEE, 1992 pp 41-45), it is reported that the etching can be conducted at a suitable etching rate when ammonium chloride is used in the etching. However, etching residues are formed, and this process cannot be used.